The present invention relates to an interference signal canceling method, a receiver using the same and a digital multi-carrier communication system comprising such a receiver. More particularly, it relates to an interference signal canceling method which compensates the transmission performance degradation due to co-channel or similar interference signals from other adjacent cells in digital mobile radio communication or other adjacent transmitter in digital wireless communication or in digital broadcasting system and a receiver and a communication system using such an interference signal canceling method.
There have already been proposed several types of receivers which employ replica generators for interference canceling. They generate replicas by using transmission symbol candidates both for desired and interference signals, and transmission channel parameters corresponding to these two signals. Then, they subtract these replicas from a received signal to obtain an error signal. They multiply the square of the error signal by −1 and use the resulting signal as a metric for a maximum likelihood sequence estimation (MLSE) both for desired and inter-channel interference signals under inter-symbol interference condition.
For example, W. Van Etten has proposed a receiver using the Viterbi algorithm as a maximum likelihood sequence estimator (W. Van Etten, “Maximum Likelihood Receiver for Multiple Channel Transmission System,” IEEE Trans. on Comm. February 1976). However, this receiver is based on the assumption that the channel impulse response values are preknown. A receiver that estimates channel parameters and employs a maximum likelihood sequence estimation has been proposed by Howard E. Nicols, Arithur A. Giordano and John G. Proakis (H. E. Nichols, A. A. Giordano, and J. G. Proakis, “MLD and MSE algorithm for adaptive detection of digital signals in the presence of interchannel interference,” IEEE Trans. on Information Theory, September 1977). According to their proposal, the channel parameters are estimated and updated by an adaptation algorithm by using an estimated symbol value which is output from the maximum likelihood sequence estimator with a decision delay of several symbol duration. This receiver operates well when the radio channel has relatively slow time-variations. In the mobile radio channel, however, since the amplitudes and phases of desired and interference signals vary very rapidly, the estimated channel parameters with the time delay of several symbol duration represent no longer the current channel impulse response. Hence, the transmission performance is seriously degraded.
To improve the transmission performance of a receiver based on the maximum likelihood sequence estimation scheme, A. P. Clark, J. D. Harvey and J. P. Driscoll have proposed a Near-Maximum-Likelihood detection scheme as a solution to the poor channel parameter estimation due to the fixed estimation delay which poses a serious problem in the adaptive maximum likelihood sequence estimation receiver (A. P. Clark, J. D. Harvey and J. P. Driscoll, “Near-Maximum-Likelihood detection processes for distorted digital signals,” Radio & Electronics Engineer, Vol. 48, No. 6, pp. 301–309, June 1978).
Moreover, A. P. Clark has proposed an FDM (Frequency Division Multiplexing) system that transmits two signals over the same frequency channel through utilization of the Near-Maximum-Likelihood detection scheme (U.S. Pat. No. 4,862,483). In this system, however, the number of the transmission signal sequence candidates (first vector) to be stored in a memory and the number of the sets of transmission channel parameters (vectors) corresponding to the first vector are large. Each first vector is extended into a second vector by adding a further component representing a respective combination of data symbols that could be received at the sample instant. New signal sequence candidates (first vectors) are selected among the extended signal sequence candidates (i.e. second vectors) in a highest likelihood order. When the likelihood of the transmission signal sequence candidate (first vector) who has the highest likelihood is extremely higher than that of the other signal sequence candidates (first vectors), the likelihood order of the extended sequence candidates (second vectors) depends dominantly on the likelihood value of the first vector. Hence, there is almost no possibility of other first vectors being selected. This receiver can no longer be considered as a maximum likelihood detector.
H. Yoshino, K. Fukawa and H. Suzuki have proposed, as an interference signal canceling method, a receiver using a transmission parameter estimation scheme suitable for the maximum likelihood sequence estimation which keeps high-speed, precise track of the fast fading or fast changing mobile radio channel (U.S. Pat. No. 5,537,443). An interference canceller of this scheme cancels both inter-symbol interference and co-channel interference, but the number of the states in the Viterbi algorithm increases exponentially as the maximum excess delay caused by multi-path propagation increases. When the signal delay exceeds the maximum signal delay considered in the Viterbi algorithm, the maximum likelihood sequence estimator does not work, and the transmission performance is seriously degraded.
On the other hand, S. B. Weinstein and P. M. Ebert have proposed a modulation and demodulation scheme to overcome the effect of the inter-symbol interference which is denoted orthogonal frequency division multiplexing (OFDM) and uses the discrete Fourier transform (S. B. Weinstein and P. M. Ebert, “Data transmission by frequency division multiplexing using the discrete Fourier transform,” IEEE Trans. on Comm., October 1971). A receiver of this scheme does not cancel co-channel interference, and hence possesses the drawback that it does not operate in a high co-channel interference environment.
A description will be given first, with reference to FIG. 1A and FIG. 1B, of a conventional scheme of the orthogonal frequency division multiplexing (OFDM) data transmission that has the above-said feature of avoiding the intersymbol interference. FIG. 1A shows the transmitter scheme for an OFDM data transmission system. This transmitter is made up of: a serial to parallel (S/P) converter 1 which converts serial data stream to a set of L parallel data streams; parallel baseband modulators 2 to which the data in each sub-channel are fed; an inverse discrete Fourier transform (IDFT) device 3, whose outputs correspond to L channel transmitted signals with carrier frequencies f0, f1, . . . , fL−1, where the frequency difference between adjacent channels is Df and the overall bandwidth W of the L modulated carriers is L Df; a cyclic extension device 4 which adds a cyclic prefix (guard interval) in order to avoid the effect caused by inter-block interference; a parallel to serial converter (P/S) 5 to output time domain signal; a digital-to-analog (D/A) converter 6 which converts digital signal to an analog waveform; and a low pass filter 7 which limits the frequency spectrum.
FIG. 1B shows the receiver scheme for an OFDM data transmission system. This receiver is made up of a low pass filter 10 which band-limits the received signal; an analog-to-digital (A/D) converter 11 which converts the analog received signal to digital form; a serial-to-parallel (S/P) converter 12 which converts the serial received signal to L parallel data streams; a remove cyclic extension device 13 which removes the cyclic prefix in order to remove the effect of time domain inter-symbol interference; a discrete Fourier transform (DFT) device 14 whose output are L sub-carrier channels; L baseband demodulators 15 which demodulate baseband received symbols to digital data bits; and a parallel-to-serial (P/S) converter 16 which recombines digital data bits to a serial data bit sequence. An advantage of the OFDM data transmission system is that OFDM reduces the effects of intersymbol interference (ISI) by lowering the symbol rate for each sub-carrier. Particularly, for the application of high bit rate digital signal transmission, OFDM also can eliminate the effect of ISI by adding a cyclic prefix, the length of which is set to be greater than the maximum excess delay of the transmission channel. When the received signal contains an interference signal from another transmitter, the interference signal still remains on each of the sub-channels so that the transmission performance is seriously degraded. In a cellular mobile communication system in which each cell may sometimes receive a co-channel interference signal from an adjacent cell, or in wireless local or wide area network in which the same frequency channel is reused (e.g. space division multiple access (SDMA) systems), there is a strong demand for canceling the influence of the interference. In a digital broadcasting system, such as digital audio broadcasting (DAB) or digital video broadcasting (DVB), the receiver at the fringe of a broadcasting service area suffers a co-channel interference from the transmitter in an adjacent broadcasting service area if in both service areas the same frequency channels are used. Thus, there is also a strong demand for canceling the interference signal.